The penis is not muscular. The main structures of the penis include the corpus cavernosum and the urethral corpus cavernosum. The corpus cavernosum is located on the dorsal side of the penis, one on each side. When sexually stimulated, the corpus cavernosum will rapidly fill with blood, and at the same time compress the white membrane on the surface of the corpus cavernosum, which contains the draining veins, and the venous return is blocked after being compressed, so that the penis is able to maintain an erection. The urethral spongiosum is located in the ventral side of the penis, the urethra passes through it, the urethral spongiosum is not congested when sexually excited, the urethra will not be pressed closed for ejaculation to provide favorable conditions. In addition to the corpus cavernosum, the penis has arteries, veins and nerves. The arteries are located deeper and are responsible for providing blood supply to the penis, the veins are located superficial and are responsible for the return of blood to the penis, and the nerve trunk is distributed in the dorsal side of the penis and is responsible for the sensation of the penis, because there is no muscle, there is no motor function of the penis, and the penis can be slightly active in fact, it is actually due to the contraction of the levator ani muscle that pulls on the skin when it is caused by the contraction of the levator ani muscle.